Cryogenic freezers have been used for many years to rapidly cool and freeze various articles such as food on a continuous basis.
Cryogenic freezers come in many forms, one being the tunnel-type freezer, which is adequately described in U.S. Pat. No. 4,229,947. The tunnel-type freezers can employ various cryogens, among them liquid nitrogen, liquid carbon dioxide and liquid air.
Another type of cryogenic food freezer is the spiral food freezer named because the conveyor belt is arranged in either an ascending or descending spiral path within an enclosure. Examples of this type freezer are disclosed in U.S. Pat. Nos. 4,023,381; 4,078,394; 4,103,507; 4,103,768; 4,164,129; 4,271,683; 4,324,110 and 4,356,707. In the ascending type freezer the product is introduced through a lower opening in the chamber and conducted along the spiral conveyor in a helical path to an exit disposed near the top of the insulated housing or chamber.
Existing cryogenic spiral freezers have no method to prevent the loss of cold gas from the lower opening. Some cryogenic spiral freezers incorporate a flexible curtain at the lower opening in an attempt to reduce this loss of refrigeration. However, the flexible curtains can reduce, but can not eliminate this loss. The flexible curtains are not durable and are easily damaged. Further, the flexible curtain can interfere with food products placed on the conveyor belt.
The conventional spiral freezers have a further disadvantage when production is interrupted, such as during a lunch break. To maintain the operating temperature, the flow of cryogenic fluid injected into the freezer is reduced to about 8% of the normal flow. Under these conditions the flow out of the lower opening is much greater than the volume of cryogenic fluid vaporized within the freezer. As a result, warm room air will flow into the upper opening. The water vapor contained in the room air condenses and freezes causing an accumulation of frost and ice within the freezer, as well as a significant heat input into the freezer.
In operation when the insulated enclosure of the spiral freezer is at operating temperature, because of the vertical distance between the openings of the freezer, cold dense gas inside the enclosure will spill out of the lower opening causing a significant loss of refrigeration.
The existing spiral freezers employing liquefied carbon dioxide as the cryogen lose the most significant amount of refrigeration from the cold gas spilling out of the lower opening because of the greater density of gaseous carbon dioxide. However, these spiral freezers have an oversize exhaust fan with an exhaust hood at the lower opening to prevent the carbon dioxide gas from accumulating in the processing room. Thus, the exhaust hood prevents the freezer operator from observing the loss of cold gas.
One attempt at solving this serious problem is disclosed in U.S. Pat. No. 4,739,623. The apparatus shown in the '623 patent attempts, by creating a negative pressure inside of the cabinet, to prevent egress of cryogen from the cabinet or ingress of ambient air into the cabinet through the inlet opening. The problem with the freezer of the '623 patent resides in the fact that since a sensor or thermocouple is placed in the exhaust hood disposed adjacent the inlet opening of the freezer, there is not a good signal for control because the temperature range of operation is very small since only a small amount of cryogen mixes with a large amount of ambient air to create a detectable condition for control. In addition, the apparatus of the '623 patent uses a pair of slideable gates in conjunction with the openings and baffles to create an effective control system. The gates must be positioned manually in order to balance and control conditions inside of the cabinet.